1. Field of the Invention
The invention is concerned with refining crude glyceride oils for the separation of impurities and in particular with the recovery of triglycerides which constitute the principal components of crude glyceride oils, and phosphatides which are also present, particularly in certain vegetable fats and which are a valuable commodity in their own right.
Fat is recovered from animal tissue by a simple rendering process but the recovery of vegetable oils is more difficult, requiring careful mechanical and heat treatment and often also solvent extraction to increase yield. From both sources a mixture of fat-soluble and fat-insoluble impurities is obtained along with the glycerides. Fat-insoluble impurities, for example fragments of oilseeds and cell tissues, are removed by mechanical means, for example settling, filtration and centrifugation by means of which solid particulate matter is separated.
Fat-soluble impurities remaining in the crude glyceride oils thus obtained vary widely in chemical composition and include proteins, gums, resins, phosphatides, hydrocarbons, ketones and aldehydes. Although certain components, notably tocopherols, vitamins and sterols, are retained as far as possible for their favourable effects in the refined oil, the removal of most fat-soluble impurities is essential, particularly where the oil is required for edible purposes and to ensure stability against oxidation and microbiological deterioration during storage. The complexity of crude glyceride oil refining is due largely to the difficulties attending their removal, but in conventional processes a major part of the process consists in subjecting the crude oil to chemical treatment with the object of converting the fat-soluble impurities to fat-insoluble substances which can than be removed by physical means as described.
With the development in comparatively recent years of high-yield extraction processes aimed at achieving maximum extractive efficiency, these operations have become if anything more difficult.
The vast bulk of the constituents of crude glyceride oils including the glycerides themselves are of course compounds containing fatty acid residues. Of these, lecithin are present in a sludge fraction, also known as phosphatides, and containing in addition sugars, free fatty and other acids. Lecithins are valuable by-products which are widely used as food emulsifiers. Their removal from glyceride oils is indeed essential for this reason. Refining to separate fat-soluble impurities involving treatment with aqueous alkali and/or acid solutions leads to the formation of tenacious emulsions in the presence of these phosphatide components, with consequent considerable loss of oil. Other emulsifying agents likely to be present are the soaps formed by neutralisation of free fatty acid present in the oil, and by saponification of the oil itself during alkali treatment.
In the separation of phosphatides from crude glyceride oils by conventional processes, the crude oil is subjected to treatment with hot water and steam after removing any solvent if this has been used to extract the oil. A substantial proportion of phosphatide is converted to a hydrated, fat-insoluble form. This may be removed from the oil by centrifugation, but the removal of phosphatides is incomplete at this stage since hydration is incomplete. Phosphatide removal, otherwise known as desliming or degumming, is not ordinarily applied to animal fats which are very low in phosphatides, although they may be given a hydration treatment for improving colour.
A further proportion of phosphatides still present in the oil is removed in a soapstock fraction formed by neutralising free fatty acid present with lye.
The remaining portion of the phosphatides in the oil is removed by heating with a mixture of soda and waterglass solutions.
The opportunities for oil loss in the refining of glyceride oils by conventional methods, particularly the removal of fat-soluble impurities, are thus considerable. In addition, both the glycerides and the phosphatides recovered may be damaged by the severe chemical treatment to which they are exposed and the temperatures at which the treatment is carried out. Chemical damage may also be incurred by the complex constituents of the oil which, while present in minor amount, contribute notably to the keepability and nutritional value of the oil, particularly tocopherols and vitamins.
The present invention aims to provide a process in which good yields of oil are obtained without the application of severe chemical treatment. This is effected by dissolving the crude oil or phosphatide-bearing material, in solution in an organic, non-alcoholic, non-acidic solvent, particularly one in which, as in miscella produced by extraction with hydrocarbons of vegetable oils, the phophatides aggregate in micelles. The solution then obtained is brought into contact under pressure with a semi-permeable membrane of such rejection characteristics that permeate and retentate fractions are obtained, the former comprising a substantially phosphatide-free solution of refined oil in the solvent and the latter comprising lecithin. Solvent is then removed from either or both the fractions, recovering products in refined form which exhibit superior characteristics to those obtained by conventional methods.
2. The Prior Art
The separation of particles of molecular dimensions by means of dialysis through a membrane has long been known. According to British Pat. No. 660,017, components of fatty oils or of derivatives thereof containing oily constituents can be separated or extracted by dialysis through a membrane of a high polymeric substance such as rubber, which exhibits swelling with a substance to be dialysed or with a solvent used in the dialysis. In dialysis, the bodies to be separated are placed in a solvent on one side of the membrane while the pure solvent is placed on the other side. In these conditions, the solutes diffuse through the membrane at different rates, a series of successive enrichments then being possible as described in British Pat. No. 224,252 for the fractional dialysis of butters, fats and oils.
Dialysis is now of comparatively little industrial significance except possibly in the form of electrodialysis in which the selective movement of diffusing molecules is promoted by electrical potential, or in clinical applications.
In the separation of molecular species by membrane filtration under pressure, the osmotic pressure generated by solute molecules in a solution is overcome by an opposing pressure applied to the solution on one side of a semi-permeable membrane, with the effect that the smaller molecules pass through the membrane while bigger molecules are retained, thus concentrating solute molecules and, in the case of aqueous solution, hydrated ionic species. Where two or more solute systems coexist in solution, then provided that a membrane of suitable permeability is selected, an enrichment of the larger molecules occurs in the retentate compared with the smaller.
The essential differences between reverse osmosis and dialysis have already been recognised. Dialysis is an irreversible charge leading to increase in entropy, and dependent for its driving force upon a concentration gradient across the membrane, whereas reverse osmosis is effected by the external application of enery (Van Oss, Progress in Separation and Purification 3, 97,1970). Reverse osmosis therefore requires a membrane mechanically resistant to the applied pressure, whereas dialysis requires a concentration gradient across the membrane. For continuity therefore, dialysis requires constant replenishment of fresh solvent on the solvent side of the membrane. The rejection characteristics of a given membrane are scarcely affected by operating conditions.
In reverse osmosis it is the solvent rather than the solute which traverses the membrane and against, rather than with, the concentration gradient (Tuwiner, Diffusion and Membrane Technology, Rheinhold, 1962, p.333). Thus, dialysis results essentially in dilution of solute. Dialysis is unaffected by pressure, except to the extent that any solvent flow through the membrane due to osmosis could be stopped.
Membranes suitable for use in the invention are of two types: homogeneous membranes which are commonly of natural or synthetic rubber or polymerised hydrocarbons or siloxanes and their derivatives, and anisotropic membranes. Homogeneous membranes described hitherto have been proposed for dialysis methods of separation as in U.S. Pat. No. 3,440,264. Anisotropic membranes have been proposed for reverse osmosis in aqueous systems (Van Oss, 1960 loc. cit.), comprising a very thin skin effecting actual separation, on a much thicker porous structure giving the membrane its mechanical stability. Examples include membranes prepared from polycations and polyanions (British Pat. No. 1,881,183), polyamides (Netherlands published Patent Specification No. 6,816,328), polythene (U.S. Pat. No. 3,320,328) and acrylonitrile. Turbulent flow in aqueous reverse osmosis filtration has also been proposed. Purification of liquid hydrocarbon fuels by reverse osmosis has been proposed in U.S. Pat. No. 3,556,990 and U.S. Pat. No. 3,320,328. According to British Pat. No. 1,313,921 lecithin is sterilised by filtration under pressure through a microporous filter which retains contaminant micro-organisms. There has not been any disclosure however of purifying the constituents of glyceride oils by reverse osmosis.